In this study, the RPC density was significantly decreased postoperatively in the FLACS group, even when the RNFL thickness was not significantly changed. However, OCTA detected no statistically significant changes in the RPC density and RNFL thickness in the CPS group during the follow-up.
The reason for the changes in RPC density was not fully clear. Radial peripapillary capillaries constitute a superficial layer of capillaries with a relatively constant calibre, and these run parallel to the retinal nerve fibre layer in the peripapillary region []. Considering the unique pattern and distribution of the vessels, the RPC was considered to be particularly vulnerable to elevated IOP when compared with other retinal capillarie []. Previous studies on laser in the femtosecond pre-treatment in cataract surgery reveal that the application of docking may cause a transient and irregular elevation of IOP [,]. Ecsedy et al. [] determined that the application of the suction ring increased patient’s IOP by up to 40 mmHg. While the IOP decreased after removing the suction ring, it remained above baseline [15]. A broad IOP fluctuation can restrict ocular blood flow and lead to retinal ischaemic and neuropathic injury [-]. Therefore, we speculate that the intense intraoperative IOP fluctuations may produce an acute injury to the retina, leading to RPC density reduction after FLACS surgery.
Another reason for the changes in RPC density might be postoperative inflammation. It has been shown that prostaglandin levels increase more noticeably after FLACS than after CPS [,]. The release of prostaglandins can lead to postoperative inflammation and consequently, the breakdown of the blood-retinal barrier []. Furthermore, thermal damage from prolonged laser treatment and mechanical microtrauma from femtosecond laser pulses may also contribute to the elevated inflammation [,]. Residual particulate lens material can stimulate inflammation and lens fragmentation following FLACS additionally cause prostaglandin-associated inflammation []. The changes in inflammatory factors and prostaglandin levels may result in the reduction of RPC density in FLACS.
Our findings showed a significant negative correlation between the changes in RPC density and FLAPT at 1 day and 1 month after surgery. However, there were no statistically significant correlations between the changes in RPC density and other clinical factors, such as: I/A time and total operating time. In this study, the FLACS required a longer surgical time and I/A time than did the CPS. Perhaps the vertical application of the femtosecond laser to the cortex below the anterior capsule can cause a structural change, which can in turn make cortical cleaving hydrodissection more difficult. The remaining cortical layer also has the potential to render I/A more difficult to perform []. Moreover, in order to reduce the intraoperative damage of blood-retinal barrier, we specifically excluded patients with moderate to severe cataract to lower the CDE. This may be why we did not observe correlations between the changes in RPC density and I/A time and total operating time.
Although there was no statistically significant change of the VD in the macular area in both groups, we observed increases in macular thickness at 1 month postoperatively and this increase remained 3 months after surgery. These changes had no effect on the improvement of visual acuity after cataract surgery, even though the differences reached the threshold of statistical significance in FLACS group instead of CPS group. Several researches reported the increase of retinal thickness after cataract removal, which was suggested to result from the breakdown of the blood-retinal barrier [,]. In the present study, the TOT was 371.47 ± 62.95 s in the group and 337.35 ± 40.10 s in the CPS group. There was a significant difference in the operating time between the two groups, which was in accordance with the comparison of I/A time [27]. Furthermore, the intraoperative CDE was 2.08 ± 1.26% in the FLACS and 2.35 ± 0.67% in the CPS group as the patients enrolled with mild cataract. In addition, phacoemulsification was completed using torsional phacoemulsification on an active-fluidics-based platform, which could maintain the relatively minor changes in IOP []. Therefore, the shorter operation time, less intraoperative injury and the relatively minor changes in IOP could contribute to the result.
One limitation of this study is that the number of included participants represents a relatively small sample size, and therefore, our results may have been influenced by the short follow-up time. Further longitudinal studies involving larger numbers of patients are thus needed.
In conclusion, OCTA provided a promising analysis of retinal vascular alterations, demonstrating the reduction of RPC density and the increase of macular thickness after FLACS. However, these changes had no effect on the improvement of visual acuity after cataract surgery.